Method for detecting a wheel of a vehicle

ABSTRACT

A rotating wheel of a vehicle is detected by evaluating the Doppler shift of a measuring beam, which is emitted by a detector unit passed by the vehicle, and is reflected by the wheel and returned in Doppler-shifted form. In a relative position to the wheel, the vehicle comprises an onboard unit, which can establish radio communication with a transceiver having a known location in the detector unit. The direction and distance of the onboard unit from the transceiver are measured on the basis of at least one radio communication between the same. The radiation direction or radiation position of the measuring beam is controlled in accordance with the measured direction and distance and taking into consideration the aforementioned relative position and location.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to European Patent Application No. 12175 290.1, filed on Jul. 6, 2012, the entirety of which is incorporatedby reference herein.

BACKGROUND

1. Technical Field

The present application relates to a method and system for detecting arotati wheel of a vehicle.

2. Background Art

Detecting vehicle wheels is of interest for numerous applications. Forexample, detecting wheels allows travel on a particular traffic area tobe recognized with certainty, for example for border monitoring purposesor for triggering particular actions, such as triggering an alarm,activating lighting, opening a gate, recording a photo for monitoringpurposes and the like. Modem traffic fee systems are also frequentlydirected to the number of axles of vehicles for fee assessment, so thatthe detection of wheels (wheel axles) can also constitute an importantbasis for imposing or checking road tolls, in particular also by way ofmobile control vehicles, which are to check the number of axles ofvehicles subject to tolls when passing them or in oncoming traffic.

It is known from DE 10 2008 037 233 A1 to detect wheels of a movingvehicle based on the different horizontal component of the tangentialspeed thereof as compared to the remaining vehicle, this tangentialspeed causing a corresponding Doppler frequency shift of a radarmeasuring beam. A radar speedometer is used for this purpose, whichirradiates the lower region of passing vehicles by way of a radar lobeand time-averages a single speed measurement signal based on thereceived frequency spectrum that is returned, the signal exhibitingsignal maxima at the locations of the wheels that are used for wheeldetection.

The applicant of the present application disclosed novel and reliablemethods for wheel detection that are particularly insusceptible tofaults based on Doppler measurements in the not previously publishedpatent applications EP 11 450 079.6, EP 11 450 080.4 and PCT/EP2012/061645.

The applicant has recognized that the best possible alignment of theDoppler measuring beam with passing vehicles is desirable so as tofurther improve the detection reliability. On multi-lane roads, or roadswith oncoming traffic, the distance at which a vehicle passes thedetector unit frequently varies drastically, either due to driving styleor the vehicle dimensions. This can result in insufficient illuminationof the wheel to be detected by the measuring beam, causing detectionerrors.

BRIEF SUMMARY

It is the object of the present application to overcome the problemsdescribed above and to create a further improved method for wheeldetection based on Doppler measurements.

This object is achieved by a method for detecting a rotating wheel of avehicle by evaluating the Doppler shift of a measuring beam, which isemitted by a detector unit passed by the vehicle, reflected by the wheeland returned in Doppler-shifted form. In a relative position to thewheel, the vehicle comprises an onboard unit, which can establish aradio communication with a transceiver having a known location in thedetector unit/The method comprises: measuring the direction and distanceof the onboard unit from the transceiver on the basis of at least oneradio communication between the onboard unit and the transceiver; andcontrolling the radiation direction or radiation position of themeasuring beam in accordance with the measured direction and distanceand taking into consideration the aforementioned relative position andlocation; wherein the relative position is measured by a stationary ormobile control device and stored in the onboard unit by way of a radiocommunication, and the relative position that is stored in the onboardunit is read out by way of a radio communication for the aforementionedconsideration.

This object is also achieved by a method for detecting a rotating wheelof a vehicle by evaluating the Doppler shift of a measuring beam, whichis emitted by a detector unit passed by the vehicle, reflected by thewheel and returned in Doppler-shifted form. In a relative position tothe wheel, the vehicle comprises an onboard unit, which can establish aradio communication with a transceiver having a known location in thedetector unit. The method comprises: measuring the direction anddistance of the onboard unit from the transceiver on the basis of atleast one radio communication between the onboard unit and thetransceiver; and controlling the radiation direction or radiationposition of the measuring beam in accordance with the measured directionand distance and taking into consideration the aforementioned relativeposition and location, wherein the relative position is measured by astationary or mobile control device and stored in a database, and therelative position that is stored in the database is looked up for theaforementioned consideration.

An aspect of the present patent application is based on using so-calledonboard units (OBUs), which in road toll and communication systems areemployed to impose tolls for the usage of locations by vehicles, forsolving the problems described above. Onboard units of this type canestablish radio communications of the dedicated short range radiocommunications (DSRC) type with roadside radio beacons (roadsideentities, RSEs) having known positions along the way, whereby they canbe located in each case by the radio coverage range of the radio beaconif the DSRC radio communication is successful. Examples of suchbeacon-based infrastructure-bound road toll systems include road tollsystems according to the CEN (European Committee forStandardization)-DSRC or ITS-WAVE (Intelligent TransportSystems-Wireless Access in Vehicular Environments) standards (IEEE802.11g). However, it is also possible for onboard units ofsatellite-based “beaconless” road toll systems, in which the onboardunits are autonomously self-locating in a global navigation satellitesystem (GNSS) and transmit the location data thereof, or toll datagenerated therefrom, to a back office, for example by way of a mobilecommunication network, to be additionally equipped with DSRC radiomodules, either for control readout purposes or as so-called “hybridOBUs”, which can cooperate both with GNSS and DSRC road toll systems.

From US 2003/0102997 A1, a collision avoidance system on the basis ofspecial OBUs is known, which transmit radar signals for radar detectionof neighboring vehicles. At the same time, the radar signals aremodulated for communication purposes communicating with OBUs ofneighboring vehicles, which, thereby, can give detailed positioninformation for easing the radar detection, for example.

The method employs the radio communication capability of the onboardunits so as to determine the geometric relationships with respect to thedetector unit, and based thereon the passing distance, based on radiocommunication when the Doppler detector unit is passed, and utilize thisdetermination, in turn, to align the Doppler measuring beam of thedetector unit. As a result, individual, adaptive and precise alignmentof the measuring beam with the wheels of a passing vehicle can beachieved, whereby all types of Doppler evaluation methods for wheeldetection can be carried out with high accuracy and precision, even withvarying vehicle distances.

So as to relate the direction and distance, which are measured based onthe radio communication between the transceiver of the detector unit andthe onboard unit of the vehicle, as precisely as possible to thedirection and distance of the measuring beam between the detector unitand the wheels of the vehicle, it is desirable to have as preciseknowledge as possible of the relative position of the onboard unit onthe vehicle with respect to the wheels of the vehicles. However, thisrelative position may vary drastically, depending on the installationsituation of the onboard unit on the vehicle. Users frequently attachonboard units to the inside of the windshield, usually in apredetermined position, such as in a corner or on the top center of thewindshield. The relative position may be measured separately for eachvehicle, so that the user does not need to follow any particularinstallation instructions or faulty installation is inconsequential. Inthe first variant, the relative position is measured by a stationary ormobile control device for this purpose and stored in the onboard unit byway of a radio communication, and the relative position that is storedin the onboard unit is read out by way of a radio communication for theaforementioned consideration. In the second variant, the relativeposition is measured by a stationary or mobile control device for thispurpose and stored in a database, and the relative position that isstored in the database is looked up for the aforementionedconsideration.

The control device, which measures the relative position, can be one ofthe geographically distributed roadside radio beacons (RSEs) of abeacon-based toll system, for example, and this measurement of therelative position of the onboard unit on a vehicle can be carried out inspecially equipped radio beacons of this toll systems. In the firstembodiment described above, the measured relative position is thenstored in the onboard unit itself and transported by the same until theread-out step is performed by the detector unit; in the secondembodiment described above, the relative position is stored for eachonboard unit or for each vehicle in a central or remote database, untilthis information is required by a detector unit and retrieved.

In the two last-mentioned variants, the relative position can bemeasured by creating a scan image of the vehicle using a scanner of thecontrol device, by radio triangulation of the onboard unit with atransceiver of the control device, and by referencing the radiotriangulation with the scan image. The radio triangulation can takeplace in particular by way of phase measurements in an antenna array ofthe transceiver of the control device.

As an alternative, the relative position could also be measuredoptically, for example by creating a frontal image of the vehicle usinga camera and optical recognition of the position of the onboard unit inthe frontal image.

The measuring of the direction and distance between the onboard unit andthe transceiver can take place by way of radio triangulation in thecourse of the radio communication between the same, in particular by wayof phase measurements in an antenna array of the transceiver of thedetector unit.

The method is suited for any type of measuring beam having a frequencythat is subject to a Doppler effect-related frequency shift uponreflection from a moving target, such as a rotating wheel in this case.The measuring beam could be a laser or ultrasonic beam, for example. Themeasuring beam may be a radar beam, and the radiation direction thereofis controlled by phase control of an antenna array of the detector unit;as an alternative, the measuring beam could be a radar beam, and theradiation position thereof could be controlled by switching betweenseveral antennas of the detector unit.

The method is also suited for cooperation with any conceivable variantof wheel detection methods based on an evaluation of the Doppler shiftof the measuring beam over a progression over time. Some variants arecharacterized in that a wheel is detected if the progression over timeof the Doppler shift indicates a jump, a rise, a drop or a spreadspectrum above a respective threshold value, and combinations of thesevariants are also conceivable.

The method is suited both for stationary and for mobile detector units.The detector unit may be configured as a control vehicle, so that themethod can be employed, for example, so as to check vehicles in oncomingtraffic, or vehicles on neighboring lanes in the same driving direction,and detect the wheels of the same.

The method is also suited for any type of radio communication that theaforementioned onboard units can carry out, for example also for mobileradio communication in terrestrial mobile communication networks.However, radio communication within the framework of beacon-supportedroad toll systems may be performed according to the CEN-DSRC or ITS-WAVEstandards.

Further features and advantages, as well as the structure and operationof various embodiments, are described in detail below with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

Embodiments will be described in more detail hereafter with reference tothe accompanying drawings. In the drawings:

FIGS. 1 a to 1 d are exemplary idealized Doppler shift progressions overtime at various angular positions of a Doppler measuring beam relativeto a wheel, according to embodiments.

FIGS. 2 a and 2 b are two variants of the method based on exemplary beampaths between a control vehicle and a vehicle to be checked, as seen inthe driving direction, according to example embodiments.

FIG. 3 shows various variants of the method based on a schematic topview onto a road section comprising a stationary control device, adetector unit configured as a control vehicle, and a vehicle to bechecked in consecutive phases of the method, according to exampleembodiments.

FIGS. 4 a and 4 b show the geometric fundamentals of the method based ona top view (FIG. 4 a) and a front view in the driving direction (FIG. 4b) of the control vehicle and the vehicle in oncoming traffic to bechecked, according to example embodiments.

Embodiments will now be described with reference to the accompanyingdrawings.

DETAILED DESCRIPTION

The present application relates to a method and system for detecting arotating eel of a vehicle by evaluating a Doppler shill of a measuringbeam, which is emitted by a detector unit passed by the vehicle,reflected by the wheel and returned in Doppler-shifted form

FIGS. 1 a-1 d, 2, and 3 show the principle of detecting a rotating wheel1 of a vehicle 2, which is moving on a road 3, more precisely on a lane3′ thereof, in a driving direction 4. The wheel detection method iscarried out by way of or in a detector unit 5, which in the exampleshown is mobile and configured as a control vehicle. The detector unitor the control vehicle 5 is moving on a second lane 3″ of the road 3 ina driving direction 4′, for example, which may be—but notnecessarily—anti-parallel to the driving direction 4 of the vehicle 2 tobe checked. Of course, the detector unit 5 could also be stationary, forexample it could be set up at the roadside of the road 3 or lane 3′.

The detector unit 5 transmits a measuring beam 6, for example anultrasonic, a LIDAR or a radar measuring beam, to the vehicle 2, or thewheels 1 thereof, upon passing so as to detect the wheels 1. It isapparent from the side views of FIGS. 1 a to 1 d that the measuring beam6 can be directed at the wheel 1 from the front (FIG. 1 a), obliquelyfrom the front and above (FIG. 1 b), from above (FIG. 1 c) or any otherdirection of the drawing plane of FIG. 1 at an angle α relative to thevertical. It is apparent from FIG. 2 a that the measuring beam 6, asseen looking in the driving direction 4, can be emitted by the detectorunit 5 at differing angles β relative to the horizontal, for examplefrom a—radiation position A at a predetermined height h_(s) above theroad 3, which is illustrated by four exemplary beam paths R₁, R₂, R₃ andR₄ of the measuring beam 6. It is apparent from FIG. 2 b that themeasuring beam 6 can also be emitted, for example, from radiationpositions A₁, A₂, A₃, A₄ at various heights h_(s1), h_(s2), h_(s3),h_(s4), for example at identical angles β relative to the horizontal.The top view of FIG. 3 shows that the measuring beam 6 can havediffering angles γ relative to the driving direction 4 (or 4′), forexample directed obliquely forward from the detector unit 5.

The detector unit 5 is a Doppler detector and, as is known in the art,evaluates the received frequency of the measuring beam 6 that isreflected by the vehicle 2 or the wheels 1 thereof, wherein the Dopplereffect-related frequency shift Δf between the emitted and reflectedmeasuring beams 6 can be used to determine the component v_(p), aslocated (projected) in the direction of the measuring beam 6, of therelative speed v of the vehicle 2, or the tangential speed v_(t) of thewheel 1, at the respective point P of the—impingement region of themeasuring beam 6. In the right halves of FIGS. 1 a to 1 d, theprogression over time of this Doppler effect-related frequency shift, inshort Doppler shift, Δf is plotted against the time t.

If the measuring beam 6 is emitted parallel to the plane of the road 3(α=90°, β=0°, γ≠90°), the Doppler shift progression over time shown inFIG. 1 a is obtained, showing a sudden increase 9 as soon as themeasuring beam 6 impinges on the body of the vehicle 2 traveling at thespeed v, and an additional jump 10 during passage of the wheel 1. If themeasuring beam 6 impinges on the wheel 1 or vehicle 2 slightly obliquelyfrom above (0<α<90°, 0<β<90°, 0<γ<180°), the Doppler shift progressionover time shown in FIG. 1 b is obtained, showing an increase (or a drop,depending on the viewing and passage direction) 11 during passage of awheel 1. A beam direction obliquely from above with α=0°, β<β≦90° andγ=90° results in the increases (or drops, depending on the viewingdirection) shown in FIG. 1 c, which have been adjusted for the speed vof the vehicle 2.

FIG. 1 d shows that, in the case of a beam cross-section of themeasuring beam 6 that is expanded in real terms and does not have theideal-punctiform shape in the impingement region 12 of the measuringbeam 6 onto the wheel 1 or the vehicle 2, always a superposition of thediffering (tangential) speeds or projected speeds v_(p) caused bydiffering points P in the impingement region 12 occurs, which duringpassage of a wheel 1 results in a received frequency mixture, which isto say a fragmentation or spread F of the Doppler shift progression overtime, which is larger than the spectrum spread F₀ that occurs merelyduring passage of the body of the vehicle 2. Such a spread spectrum Fcan likewise be established as a criterion for the occurrence of a wheel1.

The occurrence of a wheel 1 on a passing vehicle 2 can thus be detected,for example, by a sudden frequency change 10, an increase or a drop 11and/or a spread spectrum F, each exceeding a predetermined thresholdvalue.

The detector unit 5 can be of any type known in the art for thedescribed Doppler evaluation and detection, either using a continuous,modulated or pulsed measuring beam 6. In the case of a continuousmeasuring beam 6, a Doppler frequency shift between the naturalfrequencies (“carrier frequencies”) of the emitted and of the reflectedmeasuring beam 6 can be determined by way of interference measurement,for example. In the case of a pulsed or modulated measuring beam, aDoppler shift between the pulse rates or modulation frequencies of theemitted and of the reflected measuring beam 6 can be measured. Suchnatural, carrier, pulse or modulation frequencies are understood to beincluded in the terms “emitted frequency” of the measuring beam 6 and“received—frequency” of the reflected measuring beam 6 that are usedhere, which is to say the term ‘received frequency’ comprises anyfrequency of the measuring beam 6 that can be influenced by a Dopplereffect.

Based on the exemplary paths R₁ to R₄ of the measuring beam 6, usingdiffering radiation directions β₁ to β₄ (FIG. 2 a) or differingradiation positions A₁, A₂, A₃, A₄ (FIG. 2 b) and various lateralpositions 13 of the vehicle 1 with respect to the detector unit 5 (arrow14), FIGS. 2 a and 2 b show that there are radiation directions β orradiation positions A for which the measuring beam 6 misses the vehicle2 and/or the wheels 1 thereof. The method that is described hereafter isused to prevent this.

The method is based on the use of onboard units (OBUs) 15, which arecarried by a respective vehicle 2, so as to allow the vehicle toparticipate in a road toll or communication system. Because thedetection of wheels 1 of a vehicle 2 is frequently used as a basis fortoll assessment, notably for road toll systems, the OBUs 15 can, at thesame time, be used in such road toll systems for the purposes that areset out herein.

FIG. 3 shows sectional views of a road toll system 16, comprising aplurality of geographically distributed control devices 17 (only onebeing shown), which are set up along the road 3 at mutual distances fromeach other, for example. The control devices 17 are connected to a backoffice 19 of the road toll system 16 by way of data lines 18. The roadtoll system 16, and more particularly the control devices 17 thereof,impose tolls (charge fees) for the location usages by vehicles 2, forexample the driving on the road 3.

For this purpose, the control devices 17 can be configured, for example,as radio beacons comprising a transceiver 21 that is arranged on agantry 20 and a connected beacon processor 22 and they can carry outdedicated short range communication (DSRC) with the OBU 15 of a passingvehicle 2 by way of the transceiver 21. For example, the DSRC radiocommunication 23 can result in a toll transaction, which is reported tothe back office 19 via the beacon processor 22 and the data connection18 and/or is stored in the OBU 15.

The control devices (radio beacons) 17, the OBUs 15 and the internaltransceivers thereof for carrying out the DSRC radio communication 23can be composed according to all known DSRC standards, notably CEN-DSRC,ITS-G5 or WAVE (wireless access in vehicular environments). In thecourse of the passing of a radio beacon 17, each DSRC radiocommunication 23 can, for example, debit a current account in the backoffice 19 and/or the OBU 15 with a particular usage fee and thenconstitutes a “debit transaction”; however, the DSRC radio communication23 can also form identification, maintenance, or software updatetransactions or the like within the framework of the road toll system16.

The DSRC radio communication 23 can, in particular, also be used forwirelessly, polling (reading out) data that is stored in the OBUs 15,such as master data, identification data, transaction data, recordingdata and the like. Such wireless polls 23 can originate not only fromthe stationary control devices or radio beacons 17, but also from“mobile” radio beacons 17 in the form of a detector unit 5 configured asa control vehicle. In other words, the detector unit 5 can also functionas a radio beacon 17, and in addition, a radio beacon 17 can converselyfunction as a detector unit 5. Everything that is described with regardto the DSRC communication capability of the radio beacon 17 thereforealso applies to the detector unit 5, which for this purpose is equippedwith a dedicated transceiver 24, and vice versa.

Wireless polling of OBUs 15 via DSRC radio communication 23 canadditionally be carried out in global navigation satellite (GNSS) roadtoll systems 16, in which, instead of a network of terrestrial radiobeacons 17, the OBUs 15 in each case are autonomously self-locating byway of a GNSS receiver and transmit the locations thereof, or the tolltransactions determined based thereon, to the back office 19, forexample by way of the radio beacon network or a separate mobilecommunication network. Again, the OBUs 15 can be equipped with DSRCtransceivers for wireless polling by radio beacons (control devices) 17or control vehicles (detector units) 5. The method described here, andthe detector unit 5 discussed here, are therefore suited for cooperatingboth with beacon-based and with satellite-based road toll systems 16.

A radio communication 23 between the transceiver 24 of the detector unit5 and the internal transceiver (not shown) of the OBU 15 is thereafterused to determine the distance between the detector unit 5 and thevehicle 2 to be checked and to control, based thereon, the radiationdirection β and/or radiation position A of the measuring beam 6 of thedetector unit 5. A detector unit 5 is used for this purpose, themeasuring beam 6 of which can also be controlled accordingly: if thedetector unit 5 operates based on a Doppler radar, this can be done, forexample, by mechanically pivoting or adjusting a directional antenna 25,by way of which the measuring beam 6 is emitted and received. instead ofa directional antenna 25, it is also possible to use an antenna array,the radiation direction of which can be adjusted by appropriate phasecontrol, as is known in the art. As an alternative or in addition, thedetector unit 5 could also comprise a group 25′ of several antennas orantenna arrays, which are arranged mutually spaced from each other, forexample at the heights h_(s1) to h_(s4) and can be switched so as toachieve differing radiation positions A₁ to A₄. The beam paths R₁ to R₄from the various radiation positions A₁ to A₄ can also have differingangles β. In the case of a detector unit 5 that operates based on aDoppler LIDAR, the radiation direction β and/or the radiation position Aof the measuring beam 6 could also be varied using an appropriatearrangement of deflection mirrors, as is known in the art. In the caseof a detector unit 5 that is based on ultrasound Doppler evaluation,appropriate mechanically adjustable ultrasonic transducers orphase-controllable ultrasonic transducer arrays could be employed, orthe like.

The method is further based on the use of transceivers 24 in thedetector unit 5, which are able to measure the length of thecommunication link of a radio communication 23, which is to say thedistance z between the transceiver 24 and the OBU 15, and the directionof the OBU 15 with respect to the transceiver 24. This is shown indetail in FIGS. 4 a and 4 b.

According to FIG. 4 a, the “direction” of the OBU 15 of the vehicle 2with respect to the transceiver 24 of the detector unit 5 denotes atleast the angle δ parallel to the plane of the road 3 of the imaginaryconnecting line between the OBU 15 and the transceiver 24, relative to anormal to the driving direction 4, or more precisely to the drivingdirection 4′ of the detector unit 5.

In a first step of the method, the direction δ and distance z of the OBU15 from the transceiver 24 are measured based on radio communication 23that is carried out between the OBU 15 and transceiver 24. For thismeasurement, the transceiver 24 can comprise multiple antennas or anantenna array, for example, in which the received direction δ of a datapacket that is transmitted by the OBU 15 and received by the transceiver24 over the course of the radio communication 23 can be determined byway of propagation time and/or phase measurements. For example, thedistance z can be determined from signal propagation time measurements,or even by transmitting GNSS position measurements of the OBU 15provided by the OBU to the transceiver 24, which compares theinformation to its own GNSS position measurements.

So as to determine the passing distance between the vehicle 2 and thedetector unit 5, and in particular the location of the wheels 1 relativeto the radiation position A of the measuring beam 6, based on thedistance z and the direction δ, having knowledge of both theinstallation location of the OBU 15 on the vehicle 2 and the location ofthe transceiver 24 with respect to the radiation position A of thedetector unit 5 is required.

The installation location of the OBU 15 on the vehicle 2 is only ofinterest with regard to the relative position R of the OBU 15 withrespect to the wheels 1, and in particular with regard to the lateraldistance b of the OBU 15 from the outside of the wheel 1 and theinstallation height h_(b) of the OBU 15 above the road 3 in relation tothe height h_(r) of the wheel 1 above the road 3; the installationlocation of the OBU 15 in the driving direction 4 on the vehicle 2 isnot relevant here.

On the side of the detector unit 5, the location L of the transceiver 24with respect to the radiation position A of the measuring beam 6 can becomputed based on the lateral distance a of the transceiver 24 from theradiation position A and based on the difference h_(a)-h_(s) between theinstallation height h_(a) of the transceiver 24 and the installationheight h_(s) of the radiation position A above the road 3.

The location L or (a, h_(a)−h_(s)) of the transceiver 24 in the detectorunit 5 is always known. The relative position R or (b, h_(b)−h_(r)) ofthe OBU 15 on the vehicle 2 can vary depending on the installationlocation, for example if the user himself attaches the OBU 15 indiffering positions on the inside of the windshield. According to afirst variant of the method, the relative position R of the OBU 15 onthe vehicle 2 is predetermined for the user, which is to say he mustattach the OBU 15 in a predetermined relative position in accordancewith the type of his vehicle 2. The relative position R can then beregarded as known; for example, it can be obtained from vehicle-specifictables.

In an alternative embodiment, the relative position R of the OBU 15 onthe vehicle 2 is measured, and the measurement result is stored eitherin the OBU 15 itself or in a database in the road toll system 16, forexample one of the radio beacons 17, a proxy processor of the road tollsystem or the back office 19. This measuring process can be carried outby the control device or radio beacon 17, for example, which isschematically indicated in FIG. 3. The radio beacon 17 can comprise ascanner 26 for this purpose, for example a laser scanner, whichgeometrically scans the vehicle 2 during passage; at the same time,using radio triangulation, for example by way of phase measurements onmultiple antennas or in an antenna array, the transceiver 21 of theradio beacon 17 can measure the relative position R, referencing theradio triangulation with the scan image of the scanner 26. Instead of ascanner 26, a camera could be used, for example, which creates a frontalimage of the passing vehicle 2, in which again the relative position Rof the OBU 15 is referenced by radio triangulation by way of thetransceiver 21, or with an OBU 15 that is attached in particular to theinside of the windshield being detected in the frontal image by opticalrecognition of the relative position R.

The relative position R thus measured by a control device 17 connectedupstream of the detector unit 5 can then be stored in the OBU 15 by thetransceiver 21, for example by way of radio communication 23, or can bestored in a database of the road toll system 16, for example in theradio beacon 17 or, via the data path 18, in the back office 19.

In the further course of the method, if these values are required by thedetector unit 5 for controlling the measuring beam 6, the relativeposition R can thus either be received from the OBU 15 via a radiocommunication 23 or can be requested for a—particular OBU 15 via a radiodata channel 27 from the database of the radio beacon 17 or the backoffice 19, where the appropriate information is looked up. It is alsopossible to provide a separate intermediate storage unit or proxyprocessor in the road toll system 16 for this purpose, which is able toprovide the required data to the detector unit 5 especially quickly; theintermediate storage unit can be stationary or mobile and retrieve datafrom the aforementioned database on a regular basis, for example once aday, so as to make this available to the detector unit 5.

Having knowledge of the location L (a, h_(a)−h_(s)) and of the relativeposition R (b, h_(b)−h_(r)), it is then possible, using geometriccalculations, to compute the radiation direction, in particular theangle β thereof, and/or the radiation position A, in particular theheight h_(s) thereof, for the measuring beam 6 of the detector unit 5,for example in the manner described below.

Based on the distance z and direction δ the distance y that is projectednormal to the driving direction 4 follows from

y=z cos δ  (1)

and the distance x, which is projected parallel to road 3 or horizontal,between the OBU 15 and the transceiver 24 follows from

x=√{square root over (y²−(h _(b) −h _(a))²)}  (2)

Having knowledge of the transverse distances a, b from the receivedrelative position R and the known location L, the transverse distance cbetween the radiation position A and the wheel 1 follows from

c=x−b−a  (3)

The vertical distance between the radiation position A at the heighth_(s) and an impingement point P on the wheel 1, which is located at aheight F, in percentage terms, of the wheel height h_(r), for exampleF=70%, is

d=h _(s) −F·h _(r)  (4)

The desired radiation direction β, specifically for the embodiment ofFIG. 2 a having a variable radiation direction β, thus follows from

$\begin{matrix}{\beta = {\arctan \; \frac{d}{c}}} & (5)\end{matrix}$

As an alternative, the desired radiation position A at a desiredradiation height h_(s), specifically for the embodiment of FIG. 2 bhaving variably high radiation positions A₁ to A₄, follows from

h _(s) =F·h _(r) +c·tan β  (6).

The radiation direction—which in the example shown is represented in asimplified manner by the angle β, although generally one or more of theangles α, β, γ can be covered—and the radiation position A—which in theexample shown is represented in simplified form by the height h_(s),generally the radiation position A can also be established in bothremaining spatial directions—can thus be computed by measuring thedirection γ and the distance z between the onboard unit 15 and thetransceiver 24.

Of course, the detector unit 5 can be implemented both in the shownmobile form as a control vehicle, and in stationary form, for exampleusing existing radio infrastructure, such as WAVE or DSRC radio beaconsof a road toll system or WLAN (wireless local area network) radiobeacons or a roadside Internet infrastructure. This allows existingtransceiver parts of WLAN, WAVE or DSRC radio beacons, for example, tobe used as the transceiver part of a Doppler detector unit 5. The methodcan thus be implemented, for example, as a software application thatruns on a conventional mobile or stationary WLAN, WAVE or DSRC controldevice or radio beacon.

Previously, it was assumed that the emitted frequency of the measuringbeam 6 is constant, which is to say the progression over time thereof isa constant progression. However, it is also possible for the detectorunit 5 to emit a measuring beam 6 having an emitted frequencyprogression that is not constant over time, for example in the case offrequency hopping methods, in which the frequency changescontinually—according to a predetermined or known pattern. The plottedreceived frequency (mixture) progressions over time of FIGS. 1 a to 1 dare plotted relative to the previously known progression over time ofthe emitted frequency of the measuring beam 6—either in constant orchanging form—which is to say referenced or standardized thereto, sothat the effect of known emitted frequency progressions over time can becompensated for.

CONCLUSION

The invention is thus not limited to the shown embodiments, butencompasses all variants and modifications that are covered by the scopeof the accompanying claims. While various embodiments have beendescribed above, it should be understood that they have been presentedby way of example only, and not limitation. It will be apparent topersons skilled in the relevant art that various changes in form anddetail can be made therein without departing from the spirit and scopeof the embodiments. Thus, the breadth and scope of the describedembodiments should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

What is claimed is:
 1. A method for detecting a rotating wheel of avehicle by evaluating the Doppler shift of a measuring beam, which isemitted by a detector unit passed by the vehicle, reflected by the wheeland returned in Doppler-shifted form, wherein, in a relative position tothe wheel, the vehicle comprises an onboard unit, which can establish aradio communication with a transceiver having a known location in thedetector unit, the method comprising: measuring the direction anddistance of the onboard unit from the transceiver on the basis of atleast one radio communication between the onboard unit and thetransceiver; and controlling the radiation direction or radiationposition of the measuring beam in accordance with the measured directionand distance and taking into consideration the aforementioned relativeposition and location; wherein the relative position is measured by astationary or mobile control device and stored in the onboard unit byway of a radio communication, and the relative position that is storedin the onboard unit is read out by way of a radio communication for theaforementioned consideration.
 2. The method according to claim 1,wherein the measurement of the relative position is carried out bycreating a scan image of the vehicle using a scanner of the controldevice, by radio triangulation of the onboard unit with a transceiver ofthe control device, and by referencing the radio triangulation with thescan image.
 3. The method according to claim 2, wherein the radiotriangulation takes place by way of phase measurements in an antennaarray of the transceiver of the control device.
 4. The method accordingto claim 1, wherein the measurement of the relative position is carriedout by creating a frontal image of the vehicle using a camera and byoptical recognition of the position of the onboard unit in the frontalimage.
 5. The method according to claim 1, wherein the direction anddistance of the onboard unit from the transceiver are measured by radiotriangulation over the course of the radio communication between theunit and the transceiver.
 6. The method according to claim 5, whereinthe radio triangulation takes place by way of phase measurements in anantenna array of the transceiver of the detector unit.
 7. The methodaccording to claim 1, wherein the measuring beam is a radar beam, andthe radiation direction thereof is controlled by phase control of anantenna array of the detector unit.
 8. The method according to claim 1,wherein the measuring beam is a radar beam, and the radiation positionthereof is controlled by switching between a plurality of antennas ofthe detector unit.
 9. The method according to claim 1, wherein a wheelis detected when a progression over time of the Doppler shift indicatesa jump, an increase, a drop or a spread spectrum above a respectivethreshold value.
 10. The method according to claim 1, wherein thedetector unit is configured as a control vehicle.
 11. A method fordetecting a rotating wheel of a vehicle by evaluating the Doppler shiftof a measuring beam, which is emitted by a detector unit passed by thevehicle, reflected by the wheel and returned in Doppler-shifted form,wherein, in a relative position to the wheel, the vehicle comprises anonboard unit, which can establish a radio communication with atransceiver having a known location in the detector unit, the methodcomprising: measuring the direction and distance of the onboard unitfrom the transceiver on the basis of at least one radio communicationbetween the onboard unit and the transceiver; and controlling theradiation direction or radiation position of the measuring beam inaccordance with the measured direction and distance and taking intoconsideration the aforementioned relative position and location, whereinthe relative position is measured by a stationary or mobile controldevice and stored in a database, and the relative position that isstored in the database is looked up for the aforementionedconsideration.
 12. The method according to claim 11, wherein themeasurement of the relative position is carried out by creating a scanimage of the vehicle using a scanner of the control device, by radiotriangulation of the onboard unit with a transceiver of the controldevice, and by referencing the radio triangulation with the scan image.13. The method according to claim 12, wherein the radio triangulationtakes place by way of phase measurements in an antenna array of thetransceiver of the control device.
 14. The method according to claim 11,wherein the measurement of the relative position is carried out bycreating a frontal image of the vehicle using a camera and by opticalrecognition of the position of the onboard unit in the frontal image.15. The method according to claim 11, wherein the direction and distanceof the onboard unit from the transceiver are measured by radiotriangulation over the course of the radio communication between theunit and the transceiver, and the radio triangulation takes place by wayof phase measurements in an antenna array of the transceiver of thedetector unit.
 16. The method according to claim 11, wherein themeasuring beam is a radar beam, and the radiation direction thereof iscontrolled by phase control of an antenna array of the detector unit.17. The method according to claim 11, wherein the measuring beam is aradar beam, and the radiation position thereof is controlled byswitching between a plurality of antennas of the detector unit.
 18. Themethod according to claim 11, wherein a wheel is detected when aprogression over time of the Doppler shift indicates a jump, anincrease, a drop or a spread spectrum above a respective thresholdvalue.
 19. The method according to claim 11, wherein the radiocommunication takes place according to the CEN-DSRC standard or theITS-WAVE standard.
 20. A detector unit configured to detect a rotatingwheel of a vehicle by evaluating a Doppler shift of a measuring beamemitted by the detector unit, reflected by the wheel, and returned inDoppler-shifted form, the detector unit comprising: a transceiverconfigured to conduct at least one radio communication with an onboardunit of the vehicle in a relative position to the wheel to measure thedirection and distance of the onboard unit from the transceiver, thetransceiver having a known location in the detector unit; and thedetector unit is configured to control the radiation direction orradiation position of the measuring beam in accordance with the measureddirection and distance and taking into consideration the relativeposition and location; and wherein the relative position is measured bya stationary or mobile control device, and for the aforementionedconsideration, the relative position is read by the detector unit fromthe onboard unit by way of a radio communication or is looked up in adatabase.